Im looking for a sensor to measure linear position between 0 and 1000mm. The sensor is for a college project so has to be as cheap as possible. I wanted to but a draw wire potentiometer but they seem to costing around 300 and my budget is under 100. I was loooking at making a string pot using a 10 turn pot, im not sure of the electronics? would it be a difficult build?

I would use a drum of about 10 cm circumfrence, note that the circum. lies part way into wire; use a measuring tapes spring return spring to rewind wire on drum. Attach a slotted disc, 10 slots, on one end of drum Use 2 optical slot detectors, 90 deg out of phase. I believe there is an explination in AACs Volume 4.
Or, can use drum & 10, or greator.turn pot, with a stronger return spring. Use an OP Amp, to adj. output V to cover 0 to 5, or as desired.
Vol IV- Ch 11- synchronous counters- last page

I would use a drum of about 10 cm circumfrence, note that the circum. lies part way into wire; use a measuring tapes spring return spring to rewind wire on drum. Attach a slotted disc, 10 slots, on one end of drum Use 2 optical slot detectors, 90 deg out of phase. I believe there is an explination in AACs Volume 4.
Or, can use drum & 10, or greator.turn pot, with a stronger return spring. Use an OP Amp, to adj. output V to cover 0 to 5, or as desired.
Vol IV- Ch 11- synchronous counters- last page

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Actually, the OP only needs one opto-detector, since they are only measuring in one direction (that, from an earlier post). The "tape measure" spring is called a "constant force" spring in the industry.

It's a ten turn pot. That's why it is labeled, "10 turns". The unlabeled pot is for overshoot compensation. If it needs to be compensated for undershoot (below zero volts), that's a bit complicated for a concept drawing with no accuracy requirements specified by the OP.

Don't you think you would need a level wind mechanism to keep the cable/wire/string on the surface of the drum (think bait casting reel)? Also, the fact that the cable is wound in a helix has to be considered when designing the drum, in addition to the diameter of the cable.
These things are expensive for a reason.

Ron, point well taken & considered. Wire, or cable. that I am looking at is the type used on drafting board sliding arm & is .o3 in, ss. Ten turns only takes .3 in, so with constant tension should lay down in one layer. If required, a simple level wind could be added; two gears. threaded rod [ screw], & traveling nut.

Our OP hasn't come back. I'm wondering if 1cm resolution would be sufficient.
I have always found that "simple" mechanisms like this are not so simple, unless you have access to a machine shop. Where do you get a ≈10cm circumference drum that will couple to the shaft on the pot? Where do you get the slotted disk? I suppose it could be printed on clear mylar. Attaching the spring might even be a challenge.

....Where do you get the slotted disk? I suppose it could be printed on clear Mylar. Attaching the spring might even be a challenge.

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There are free programs out there to create and print any encoder pattern on Mylar with a laser printer.
You don't actually have to attach the constant force spring to the cylinder. If you maintain >1.5 wraps minimum it will hold tight.

(S)He's also posted over on the Electro-Tech Forum. Someone there suggested an accelerometer. Sounds like a good, inexpensive, high-tech solution.

Re Ron H: Where do you get a drum; If the max equipment is a drill press, then a 1.5 in hole saw makes a nice 1.25 in drum, which can be filed down or built up with thin metal, ie beer can. Looks like " Gym equipment project" & this one are kaput.